Color Liquid Crystal Displays and Display Backlights

ABSTRACT

There is provided a display backlight ( 604 ), including an excitation source ( 644 ) for generating blue light ( 650 ); and a wavelength converter ( 654 ) being a unitary construction including a combination of a wavelength selective filter layer ( 658 ) bonded to a photoluminescence layer ( 660 ), where the photoluminescence layer ( 658 ) includes a green photoluminescence material and a red photoluminescence material; and where the wavelength selective filter layer ( 658 ) is transmissive to blue light and reflective to green and red light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/791,702, filed on Feb. 14, 2020, and issued as U.S. Pat. No.11,442,218 on Sep. 13, 2022, which in turn claims the benefit ofpriority to U.S. Provisional Application No. 62/806,709, filed on Feb.15, 2019, entitled “Color Liquid Crystal Displays and DisplayBacklights”, all of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to color liquid crystal displays (LCDs) and inparticular a backlight arrangements for operating color LCDs thatcomprise photoluminescence materials.

Description of the Related Art

Color LCDs find application in a variety of electronics devicesincluding televisions, computer monitors, laptops, tablet computers andsmart phones. As is known, most color LCDs comprise a LC (liquidcrystal) display panel and a white light emitting backlight foroperating the display panel. Typically the LCD/backlight comprisesmultiple layers that are stacked on one another which can be problematicbecause these components require separate manufacture and assemblythereby increasing complexity and cost of the device.

The present invention concerns improvements in and relating to colorLCDs and backlights.

SUMMARY OF THE INVENTION

Embodiments of the invention concern color LCDs that includephotoluminescence materials, for example in the form ofphotoluminescence wavelength converting layer (film), which when excitedby excitation light, typically blue light, generates white light foroperating the display. Typically, the photoluminescence wavelengthconverting layer comprises a part of the backlight. Various embodimentsof the invention concern arrangements which increase display efficacy byreducing the number of layers within the display/backlight by forming awavelength converter of unitary construction.

According to an aspect of the invention, there is provided a displaybacklight, comprising: an excitation source for generating blue light;and a wavelength converter being a unitary construction comprising acombination of a wavelength selective filter layer bonded to aphotoluminescence layer, wherein the photoluminescence layer comprises agreen photoluminescence material and a red photoluminescence material;and wherein the wavelength selective filter layer is transmissive toblue light and reflective to green and red light. It will be understoodthat “transmissive” means “at least partially transmissive” and“reflective” means “at least partially reflective”. “Bonded” or“bonding” may be “directly bonded” or “direct bonding”, meaning that thewavelength selective filter layer is directly bonded to thephotoluminescence layer. For instance, the photoluminescence layer maybe manufactured separately from the wavelength selective filter layerand then “directly bonded” thereto using, for example, a lighttransmissive polymeric material. This arrangement should be understoodas the wavelength selective filter layer being “directly bonded” or“direct bonding” to the photoluminescence layer, even in the presence ofthe intervening layer of the light transmissive polymeric material forexample. Mere stacking of layers without bonding or an air interfacebetween said layers is not encompassed within the meaning of “directlybonded” or “direct bonding” in this patent specification. Additionally,“bonded” or “bonding” may be “directly deposited” or “depositingdirectly”, meaning that the photoluminescence layer is deposited(fabricated) directly onto the wavelength selective filter layer. Forinstance, in this patent specification, “depositing directly” meansdepositing in direct contact with, in that is there is no air gapbetween the layers. There may be an intervening layer, for instance alight transmissive layer, which is bonded to the wavelength selectivefilter layer and the photoluminescence layer. Such an arrangement isstill encompassed within the meaning of “directly deposited” or“depositing directly” for the purposes of this specification.

An advantage of the photoluminescence layer being bonded to thewavelength selective filter layer is that this can increase lightemission from the backlight by eliminating an air interface between thephotoluminescence layer and wavelength selective filter layer. Such anair interface could otherwise lead to a greater probability of internalreflection occurring at the interface between the photoluminescencelayer and wavelength selective filter layer.

An important feature of the invention is the unitary construction of thewavelength converter formed from the wavelength selective filter layerand the photoluminescence layer, such that the unitary construction is acombination of the wavelength selective filter layer and thephotoluminescence layer. The provision of a unitary construction in thismanner is more cost effective than known arrangements because it doesnot require the presence of an additional layer, such as lighttransmissive layer, to which the photoluminescence layer would normallybe bound in known constructions. The absence of such an additional layeralso makes the unitary construction formed in accordance with theinvention more robust and reliable than known arrangements. Since theunitary construction has a simple and efficient design, its assembly andmanufacture as part of the display backlight is significantly faster andless prone to errors than the assembly and manufacture of knownbacklights. Further, owing to the unitary construction of the wavelengthconverter comprising the combination of a wavelength selective filterlayer bonded to the photoluminescence layer—the quantum efficiency ofthe display backlight can be superior to known arrangements and canprovide a significant reduction (20-60%) in the amount ofphotoluminescence materials required.

In some embodiments, the photoluminescence layer may be bonded to thewavelength selective filter layer by directly depositing (fabricating)the photoluminescence layer directly onto the wavelength selectivefilter layer, by for example, the process of screen printing.

The photoluminescence layer may comprise a multi-layered structurecomprising a layer of the green photoluminescence material and a layerof the red photoluminescence material. The provision of different layersof the photoluminescence materials enables different thickness of saidlayers which may make it simpler and more efficient to achieve desiredrelative intensities of red and green light generation.

When the photoluminescence layer comprises a respective layer for thegreen and red photoluminescence materials, this may provide a beneficialarrangement especially where the green and red photoluminescencematerials have different absorption efficiencies.

In some embodiments, the layer of the red photoluminescence material isin closer proximity to the wavelength selective filter layer than thelayer of the green photoluminescence material. It may be understood that“closer proximity” is used to specify that the layer of the redphotoluminescence material is proximal (i.e. a proximal layer) to thewavelength selective filter layer, while the layer of the greenphotoluminescence material is distal (i.e. a distal layer) to thewavelength selective filter layer. Such an arrangement may beparticularly beneficial when the red photoluminescence materialcomprises a manganese-activated fluoride phosphor (such as KSF) whoseabsorption efficiency is significantly lower than that of a greenphotoluminescence material. In this way, the provision of the redphotoluminescence material, such as KSF, in a respective layer proximal(adjacent) to the wavelength selective filter layer can, compared with asingle-layered structure, improve luminous efficacy of the backlight andreduce the quantity of red photoluminescence material required toachieve a comparable red emission characteristic.

A multi-layered photoluminescence layer may be fabricated by fabricatinglayer of the red photoluminescence, for example by extrusion, and thendepositing layer of green photoluminescence material onto the layer ofred photoluminescence material. The multi-layered photoluminescencelayer can then be bonded to the wavelength selective filter using forexample a light transmissive material.

Alternatively, the multi-layered photoluminescence layer can be bondedto the wavelength selective filter layer by directly depositing(fabricating), by for example screen printing, a layer of onephotoluminescence material, for example the red photoluminescencematerial, onto the wavelength selective filter and then directlydepositing (fabricating) a layer of second photoluminescence material onthe layer of the first photoluminescence material.

The multi-layered photoluminescence layer may be fabricated by bondingtogether, using a light transmissive medium for example, separatelyfabricated layers of green and red photoluminescence materials. This mayimprove the ability of the backlight to achieve relative intensities ofred and green light. The multi-layered photoluminescence layer can thenbe bonded to the wavelength selective filter using for example a lighttransmissive material.

Alternatively, or additionally, the photoluminescence layer may comprisea mixture of the green photoluminescence material and the redphotoluminescence material in a single layer/light transmissivematerial. The provision of a mixture of the photoluminescence materialswithin a single layer/light transmissive material enhances ease offabrication and may make it simpler and more efficient to achieve arelative intensity of the red and green color emissions.

It may be that the photoluminescence layer further comprises particlesof a light scattering material. Inclusion of particles of a lightscattering material within the one or more layers of thephotoluminescence layer can reduce photoluminescence material usage byincreasing scattering of light within the layer and increasing theprobability of excitation light exciting the photoluminescence material.Alternatively and or in addition the wavelength converter can furthercomprise a light diffusing layer comprising particles of a lightscattering material. It is found that the inclusion of a light diffusinglayer can reduce the amount of photoluminescence material by up to 60%.Preferably, the light diffusing layer is bonded to the photoluminescencelayer. The particles of light scattering material can be selected fromthe group comprising: zinc oxide (ZnO); silicon dioxide (SiO₂); titaniumdioxide (TiO₂); magnesium oxide (MgO); barium sulfate (BaSO₄); aluminumoxide (Al₂O₃) and combinations thereof.

It may be that at least one of the green and red photoluminescencematerials comprises a phosphor material.

The green photoluminescence material may comprise phosphor material witha general composition (M)(A)₂S₄:Eu, wherein: M is at least one of Mg,Ca, Sr and Ba; and A is at least one of Ga, Al, In, Y.

The red photoluminescence material may comprise phosphor material with ageneral composition represented by the chemical formulaMSe_(1-x)S_(x):Eu, wherein M is at least one of Mg, Ca, Sr, Ba and Znand 0<x<1.0.

The red photoluminescence material may comprise a manganese-activatedfluoride phosphor material with general composition selected from thegroup comprising: K₂SiF₆:Mn⁴⁺, K₂GeF₆:Mn⁴⁺, K₂TiF₆:Mn⁴⁺, K₂SnF₆:Mn⁴⁺,Na₂TiF₆:Mn⁴⁺, Na₂ZrF₆:Mn⁴⁺, Cs₂SiF₆:Mn⁴⁺, Cs₂TiF₆:Mn⁴⁺, Rb₂SiF₆:Mn⁴⁺,Rb₂TiF₆:Mn⁴⁺, K₃ZrF₇:Mn⁴⁺, K₃NbF₇:Mn⁴⁺, K₃TaF₇:Mn⁴⁺, K₃GdF₆:Mn⁴⁺,K₃LaF₆:Mn⁴⁺ and K₃YF₆:Mn⁴⁺.

At least one of the green and red photoluminescence materials maycomprise a quantum dot material. Use of quantum dot material can improvecolor gamut and color accuracy, and can reduce power consumption of thedisplay backlight.

The green photoluminescence material may have a peak emission wavelengthin a range from 530 nm to 545 nm.

The red photoluminescence material may have a peak emission wavelengthin a range from 600 nm to 650 nm.

The blue light may have a dominant wavelength in a wavelength range 445nm to 465 nm.

The backlight may further comprise a light guide having a light emittingface and edge faces, wherein the excitation source is configured tocouple blue light into at least one edge face of the light guide andwherein the wavelength converter is disposed adjacent to the lightemitting face of the waveguide. In such an arrangement, the light guidemay be disposed between the excitation source and the wavelengthconverter. In this way, the light guide may help distribute the bluelight generated by the excitation source more uniformly over thewavelength converter.

The backlight may further comprise a light guide having a light emittingface and edge faces, wherein the wavelength converter is disposedbetween the excitation source and at least one edge face of the lightguide. In this way, the light guide may help distribute the lightgenerated by the wavelength converter more uniformly.

The light guide may be planar. Such a form may be more compact andeasily incorporated into the display backlight. In this way, the lightguide may be planar or tapered to ensure uniform emission of light fromthe backlight.

The backlight may further comprise a brightness enhancement film.

The wavelength converter may further comprise a light transmissiveprotective layer.

In another aspect of the invention, there is envisaged a wavelengthconverter for a display backlight, the wavelength converter being aunitary construction comprising a combination of a wavelength selectivefilter layer bonded to a photoluminescence layer, wherein thephotoluminescence layer comprises a green photoluminescence material anda red photoluminescence material; and wherein the wavelength selectivefilter layer is transmissive to blue light and substantially reflectiveto green and red light. The bonding may be direct bonding, meaning thatit is directly bonded.

The photoluminescence layer may comprise a layer of the greenphotoluminescence material and a layer of the red photoluminescencematerial. The provision of different layer of the photoluminescencematerials enables different thickness of said layers which may make itsimpler and more efficient to achieve relative intensities of red, blue,green. It may be that the layer of the red photoluminescence material isin closer proximity to the wavelength selective filter layer than thelayer of the green photoluminescence material.

Alternatively, or additionally, the photoluminescence layer may comprisea mixture of the green photoluminescence material and the redphotoluminescence material. The provision of a mixture of thephotoluminescence materials may make it simpler and more efficient toachieve a single color point.

It may be that at least one of the green and red photoluminescencematerials comprises a phosphor material.

It may be that at least one of the green and red photoluminescencematerials comprises a quantum dot material.

It may be that the green photoluminescence material has a peak emissionwavelength in a range from 530 nm to 545 nm.

It may be that the red photoluminescence material has a peak emissionwavelength in a range from 600 nm to 650 nm.

The wavelength converter may further comprise a light diffusing layer.It is found that the inclusion of a light diffusing layer can reduce theamount of photoluminescence material by up to 60%.

The light diffusing layer may comprise particles of light scatteringmaterial are selected from the group consisting of: zinc oxide (ZnO);silicon dioxide (SiO₂); titanium dioxide (TiO₂); magnesium oxide (MgO);barium sulfate (BaSO₄); aluminum oxide (Al₂O₃) and combinations thereof.

The wavelength converter may further comprise a protective layer.

In another aspect, the invention encompasses a method of manufacturingthe wavelength converter described herein, comprising: providing awavelength selective filter layer; and forming a unitary construction bydepositing a photoluminescence layer onto a face of and bonding to thewavelength selective filter layer. The bonding may be direct bonding,meaning that it is directly bonded.

Such a method provides a simple and efficient way of making the unitaryconstruction. The method may be a more cost effective, reliable androbust method of manufacturing the wavelength converter. This isparticularly the case since known display backlights typically utilizean additional layer of light transmissive material for coupling with thephotoluminescence layer/material, and the absence of such an additionallayer simplifies the manufacturing process, as taught by the method ofthe present invention.

The photoluminescence layer may comprise a layer of the greenphotoluminescence material and a layer of the red photoluminescencematerial. The provision of different layer of the photoluminescencematerials enables different thickness of said layers which may make itsimpler and more efficient to achieve relative intensities of red, blue,green.

Alternatively, or additionally, the photoluminescence layer may comprisea mixture of the green photoluminescence material and the redphotoluminescence material. The provision of a mixture of thephotoluminescence materials may make it simpler and more efficient toachieve a single color point.

It may be that the photoluminescence layer is deposited by screenprinting, which provides a fast deposition method and a durable layer ofphotoluminescence material.

In another aspect, the invention contemplates a display backlight,comprising: an excitation source for generating blue light with a peakemission wavelength in a wavelength range 445 nm to 465 nm; and awavelength converter being a unitary construction comprising acombination of a wavelength selective filter layer bonded to aphotoluminescence layer, wherein the photoluminescence layer comprises agreen photoluminescence material with a peak emission in a wavelengthrange 530 nm to 545 nm and a red photoluminescence material with a peakemission in a wavelength range 600 nm to 650 nm; and wherein thewavelength selective filter layer is transmissive to blue light andsubstantially reflective to green and red light. The bonding may bedirect bonding, meaning that it is directly bonded.

The photoluminescence layer may comprise a layer of the greenphotoluminescence material and a layer of the red photoluminescencematerial. The provision of different layer of the photoluminescencematerials enables different thickness of said layers which may make itsimpler and more efficient to achieve relative intensities of red, blue,green.

Alternatively, or additionally, the photoluminescence layer may comprisea mixture of the green photoluminescence material and the redphotoluminescence material. The provision of a mixture of thephotoluminescence materials may make it simpler and more efficient toachieve a single color point.

In another aspect, the invention provides a wavelength converter for adisplay backlight, the wavelength converter being a unitary constructioncomprising a combination of a wavelength selective filter layer bondedto a photoluminescence layer, wherein the photoluminescence layercomprises a green photoluminescence material with a peak emission in awavelength range 530 nm to 545 nm and a red photoluminescence materialwith a peak emission in a wavelength range 600 nm to 650 nm; and whereinthe wavelength selective filter layer is transmissive to blue light andsubstantially reflective to green and red light. The bonding may bedirect bonding, meaning that it is directly bonded.

The photoluminescence layer may comprise a layer of the greenphotoluminescence material and a layer of the red photoluminescencematerial. The provision of different layer of the photoluminescencematerials enables different thickness of said layers which may make itsimpler and more efficient to achieve relative intensities of red, blue,green.

Alternatively, or additionally, the photoluminescence layer may comprisea mixture of the green photoluminescence material and the redphotoluminescence material. The provision of a mixture of thephotoluminescence materials may make it simpler and more efficient toachieve a single color point.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention is better understood, embodiments ofthe invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional representation of a direct-litcolor liquid crystal display (LCD) in accordance with an embodiment ofthe invention;

FIG. 2 is a schematic cross-sectional representation of a front plate ofthe color LCD of FIG. 1 ;

FIG. 3 is a schematic diagram of a unit pixel of a color filter plate ofthe color LCD of FIG. 1 ;

FIG. 4 shows the filtering characteristics, light transmission versuswavelength, for red, green and blue filter elements of a color filterplate of a color LCD display according to an embodiment of theinvention;

FIG. 5 is a schematic cross-sectional representation of a back plate ofthe color LCD of FIG. 1 ;

FIG. 6 is a schematic exploded cross-sectional representation of adirect-lit backlight of the color LCD of FIG. 1 ;

FIG. 7A is a schematic representation of a wavelength converter filterlayer;

FIG. 7B is a schematic representation of the transmission and reflectioncharacteristics of a wavelength converter filter layer;

FIG. 8 is a schematic cross-sectional representation of an edge-litcolor LCD in accordance with an embodiment of the invention;

FIG. 9 is a schematic exploded cross-sectional representation of anedge-lit backlight of the color LCD of FIG. 8 ;

FIG. 10 is a schematic cross-sectional representation of an edge-litcolor LCD in accordance with an embodiment of the invention;

FIG. 11 is a schematic exploded cross-sectional representation of anedge-lit backlight of the color LCD of FIG. 10 ; and

FIGS. 12 to 19 are schematic cross-sectional representations ofwavelength converters for use in the color LCDs of FIGS. 1, 8 and 10 .

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are directed to color LCDs including aphotoluminescence wavelength converting layer which when excited byexcitation light, typically blue light, generates white light foroperating the display. Typically, the photoluminescence wavelengthconverting layer comprises a part of the backlight. Various embodimentsof the invention concern arrangements which increase display efficacy byreducing the number of layers within the display/backlight or otherwisereduces light losses at the interface between layers of the display byfor example eliminating the air interfaces.

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration. Throughout thisspecification like reference numerals preceded by the figure number areused to denote like features.

Referring to FIG. 1 there is shown a schematic cross-sectionalrepresentation of a direct-lit light transmissive Color LCD (LiquidCrystal Display) 100 formed in accordance with an embodiment of theinvention. The Color LCD 100 comprises a LC (Liquid Crystal) DisplayPanel 102 and a Display Backlight 104. The Backlight 104 is operable togenerate white light for operating the LC Display Panel 102.

LC Display Panel

As shown in FIG. 1 , the LC display panel 102 comprises a transparent(light transmissive) Front (light/image emitting) Plate 106, atransparent Back Plate 108 and a Liquid Crystal (LC) 110 filling thevolume between the Front and Back Plates 106, 108.

As shown in FIG. 2 , the Front Plate 206 can comprise a glass plate 212having on its upper surface, that is the face of the plate comprisingthe viewing face 214 of the display, a first polarizing filter layer216. Optionally, the outermost viewing surface of the front plate canfurther comprise an anti-reflective layer 218. On its underside, that isthe face of the front plate 206 facing the liquid crystal (LC), theglass plate 212 can further comprise a color filter plate 220 and alight transmissive common electrode plane 222 (for example transparentIndium Tin Oxide, ITO).

The color filter plate 220 comprises an array of different colorsub-pixels filter elements 224, 226, 228 which respectively allowtransmission of red (R), green (G), and blue (B) light. Each unit pixel230 of the display comprises a group of three sub-pixels filter elements224, 226, 228. FIG. 3 is a schematic diagram of a unit pixel 330 of thecolor filter plate. Each RGB sub-pixel 324, 326, 328 comprises arespective color filter pigment, typically an organic dye, which allowspassage of light corresponding to the color of the sub-pixel only. TheRGB sub-pixel elements 324, 326, 328 can be deposited on the glass plate212 (see FIG. 2 ) with opaque dividers/walls (black matrix) 332 betweeneach of the sub-pixels 324, 326, 328. The black matrix 332 can be formedas a grid mask of metal, such as for example chromium, defining thesub-pixels 324, 326, 328 and providing an opaque gap between thesub-pixels and unit pixels 330. To minimize reflection from the blackmatrix, a double layer of Cr and CrOx may be used, but of course, thelayers may comprise materials other than Cr and CrOx. The black matrixfilm which can be sputter-deposited underlying or overlying thesub-pixel elements may be patterned using methods that includephotolithography. FIG. 4 shows the filtering characteristics, lighttransmission versus wavelength, for red (R), green (G) and blue (B)filter elements of a Hisense filter plate optimized for TV applications.

Referring to FIG. 5 , the back plate 508 can comprise a glass plate 534having on its upper surface (that is the surface facing the LC) a TFT(Thin Film Transistor) layer 536. The TFT layer 536 comprises an arrayof TFTs, in which there is a transistor corresponding to each individualcolor sub-pixel of each unit pixel. Each TFT is operable to selectivelycontrol passage of the light to its corresponding sub-pixel. On a lowersurface (that is the surface facing the backlight) of the glass plate534 there is provided a second polarizing filter layer 538. Thedirections of polarization of the polarizing filter 116 and 138 (FIG. 1) are aligned perpendicular to one another.

Direct-Lit Backlight

Referring to FIG. 6 , there is shown an exploded view of direct-litBacklight 604 in accordance with an embodiment of the invention. Asdescribed above, the Backlight is operable to generate and emit whitelight 640 towards the rear of the LC Display Panel for operating thecolor LCD.

As shown in FIG. 6 , the backlight 604 can comprise a direct-litarrangement comprising an array of excitation sources 644 that areprovided on the floor 646 of a light reflective enclosure 648 anddistributed over the entire surface of the display. Each excitationsource 644 is operable to generate blue excitation light 650 of dominantwavelength ranging from 445 nm to 465 nm, typically from 450 nm to 460nm. The excitation sources can comprise blue light emitting GaN LEDs.

The Backlight 604 further comprises, in order of proximity from theexcitation sources 644, a light diffusive layer 652, a wavelengthconverter 654 and a Brightness Enhancement Film (BEF) 656. The lightdiffusive layer 652 ensures uniform illumination of the wavelengthconverter 654 with blue excitation light 650.

Brightness Enhancement Film (BEF)

The Brightness Enhancement Film (BEF) 656, also known as a Prism Sheet,comprises a precision micro-structured optical film and controls theemission of white light 640 from the backlight within a fixed angle(typically 70 degrees), thereby increasing luminous efficacy of thebacklight. Typically, the BEF comprises an array of micro-prisms on alight emitting face of the film and can increase brightness by 40-60%.The BEF 656 can comprise a single BEF or a combination of multiple BEFsand in the case of the latter even greater increases in brightness canbe achieved. Examples of suitable BEFs include Vikuiti™ BEF II from 3Mor prism sheets from MNTech. In some embodiments, the BEF 656 cancomprise a Multi-Functional Prism Sheet (MFPS) that integrates a prismsheet with a diffusion film and can have a better luminous efficiencythan a normal prism sheet. In some embodiments, the BEF 656 can comprisea Micro-Lens Film Prism Sheet (MLFPS) such as those available fromMNTech.

Wavelength Converter

Referring to FIG. 6 , the wavelength converter 654 is of a unitaryconstruction and comprises a wavelength selective filter layer 658 and aphotoluminescence wavelength conversion layer 660. For the sake ofbrevity, in the following description the wavelength selective filterlayer will be referred to as the “filter layer” and thephotoluminescence wavelength conversion layer will be referred to as the“photoluminescence layer”. In this specification, a unitary constructionrefers to a wavelength converter in which the photoluminescence layer isfabricated, or deposited, on the surface of the filter layer to form asingle component. This is to be contrasted to separately fabricating thephotoluminescence layer and filter layer.

One of the important features of the invention is the unitaryconstruction of the wavelength converter formed from the filter layer658 and the photoluminescence layer 660, such that the unitaryconstruction is a combination of the filter layer 658 and thephotoluminescence layer 660. The provision of a unitary construction inthis manner is more cost effective than known arrangements because itdoes not require the presence of an additional layer, such as lighttransmissive layer, to which the photoluminescence layer would normallybe bound in known constructions. The absence of such an additional layeralso makes the unitary construction formed in accordance with theinvention more robust and reliable than known arrangements. Since theunitary construction has a simple and efficient design, its assembly andmanufacture as part of the display backlight is significantly faster andless prone to errors than the assembly and manufacture of knownbacklights. Further, experiments have confirmed that a unitaryconstruction can improve the quantum efficiency of the display backlightby approximately 10% compared with known arrangements, and provides asignificant reduction (20-60%) in the amount of photoluminescencematerials required. It may be that the red photoluminescence material isin direct contact with the wavelength selective filter layer.

Wavelength Converter—Photoluminescence Layer

The photoluminescence layer 658 contains photoluminescence materials andin operation converts blue excitation light 650 into white light 640 foroperating the LC Display Panel. More specifically, the photoluminescencelayer 658 contains blue light excitable green-emitting (Peak emissionwavelength 530 nm to 545 nm) and red-emitting (Peak emission wavelength600 nm to 650 nm) photoluminescence materials. The combination ofphotoluminescence generated red light 662, photoluminescence generatedgreen light 664 and unconverted blue excitation light 650 results in awhite light emission product 640. To optimize the efficacy and colorgamut of the display, the green- and red-emitting photoluminescencematerials are selected to match their peak emission (PE) wavelengthλ_(p) with the transmission characteristic of their corresponding colorfilter elements. Preferably, the green-emitting photoluminescencematerial has a peak emission wavelength λ_(p)≈535 nm. In order tomaximize display color gamut and efficacy, the green-emitting and/orred-emitting photoluminescence materials preferably comprise narrow-bandemitting materials having an emission peak with a FWHM (Full Width HalfMaximum) of about 50 nm of less.

The green- and red-emitting photoluminescence materials can comprisephosphor materials or quantum dots (QDs) or combinations thereof. Forthe purposes of illustration, the current description specificallyrefers to photoluminescence materials embodied as phosphor materials.The phosphor materials can comprise inorganic and organic phosphormaterials. Inorganic phosphors can comprise aluminate, silicate,phosphate, borate, sulfate, chloride, fluoride or nitride phosphormaterials. As is known phosphor materials are doped with a rare-earthelement called an activator. The activator typically comprises divalenteuropium, cerium or tetravalent manganese. Dopants such as halogens canbe substitutionally or interstitially incorporated into the crystallattice and can for example reside on lattice sites of the host materialand/or interstitially within the host material. Examples of suitablegreen-emitting and red-emitting phosphor materials are given in tables 1and 2 respectively.

TABLE 1 Example green-emitting phosphor materials Phosphor FWHM familyComposition λ_(p) (nm) (nm) Sulfide SrGa₂S₄: Eu ≈536 48-50 β-SiAlONM_(x)Si_(12-(m+n))Al_(m+n)O_(n)N_(16-n): Eu 525-545 50-52 M = Mg, Caand/or Sr Aluminate YAG Y₃(Al_(1-x)Ga_(x))₅O₁₂: Ce 500-550 ≈110Aluminate LuAG Lu₃(Al_(1-x)M_(x))₅O₁₂: Ce 500-550 ≈110 Silicate A₂SiO₄:Eu 500-550  ≈70 A = Mg, Ca, Sr and/or Ba Silicate (Sr_(1-x)Ba_(x))₂SiO₄:Eu 500-550  ≈70

TABLE 2 Example red-emitting phosphor materials FWHM Phosphor familyComposition λ_(p) (nm) (nm) Hexafluorosilicate KSF K₂SiF₆: Mn⁴⁺ ≈632 ≈10Hexafluorosilicate KTF K₂TiF₆: Mn⁴⁺ ≈632 ≈10 Selenide sulfide CSSMSe_(1-x)S_(x): Eu 600-630 50-55 M = Mg, Ca, Sr and/or Ba Selenidesulfide CSS CaSeS: Eu 610-630 50-55 Silicon-ni tride CASN CaAlSiN₃: Eu600-620 ≈75 1:1:1:3 (Ca_(1-x)-Sr_(x))AlSiN₃: Eu Silicon-nitrideBa_(2-x)Sr_(x)Si₅N₈: Eu 580-620 ≈80 2:5:8

A quantum dot (QD) is a portion of matter (e.g. semiconductor) whoseexcitons are confined in all three spatial dimensions that may beexcited by radiation energy to emit light of a particular wavelength orrange of wavelengths. QDs can comprise different materials, for examplecadmium selenide (CdSe). The color of light generated by a QD is enabledby the quantum confinement effect associated with the nano-crystalstructure of the QD. The energy level of each QD relates directly to thephysical size of the QD. For example, the larger QDs, such as red QDs,can absorb and emit photons having a relatively lower energy (i.e. arelatively longer wavelength). On the other hand, green QDs, which aresmaller in size can absorb and emit photons of a relatively higherenergy (shorter wavelength). Examples of suitable QDs can include:CdZnSeS (cadmium zinc selenium sulfide), Cd_(x)Zn_(1-x) Se (cadmium zincselenide), CdSe_(x)S_(1-x) (cadmium selenium sulfide), CdTe (cadmiumtelluride), CdTe_(x)S_(1-x) (cadmium tellurium sulfide), InP (indiumphosphide), InxGa_(1-x) P (indium gallium phosphide), InAs (indiumarsenide), CuInS₂ (copper indium sulfide), CuInSe₂ (copper indiumselenide), CuInSxSe_(2-x) (copper indium sulfur selenide), CuInxGa_(1-x) S₂ (copper indium gallium sulfide), CuIn_(x)Ga_(1-x)Se₂(copper indium gallium selenide), CuIn_(x)Al_(1-x) Se₂ (copper indiumaluminum selenide), CuGaS₂ (copper gallium sulfide) andCuInS_(2x)ZnS_(1-x) (copper indium selenium zinc selenide). The opticalproperties of the core nano-crystals in one material can be altered bygrowing an epitaxial-type shell of another material. Depending on therequirements, the core/shell nano-crystals can have a single shell ormultiple shells. The shell materials can be chosen based on the band gapengineering. For example, the shell materials can have a band gap largerthan the core materials so that the shell of the nano-crystals canseparate the surface of the optically active core from its surroundingmedium. In the case of the cadmium-based QDs, e.g. CdSe QDs, thecore/shell quantum dots can be synthesized using the formula ofCdSe/ZnS, CdSe/CdS, CdSe/ZnSe, CdSe/CdS/ZnS, or CdSe/ZnSe/ZnS.Similarly, for CuInS₂ quantum dots, the core/shell nanocrystals can besynthesized using the formula of CuInS₂/ZnS, CuInS₂/CdS, CuInS₂/CuGaS₂,CuInS₂/CuGaS₂/ZnS and so on.

As described above the wavelength converter is of a unitaryconstruction. When using inorganic phosphor materials, thegreen-emitting and red-emitting phosphors, which are in the form ofparticles, can be incorporated as a mixture in a curable lighttransmissive liquid binder material and the mixture deposited directlyas a uniform layer on a wavelength selective filter layer using forexample screen printing or slot die coating. In this patentspecification, depositing directly means in direct contact with, in thatis there is no air gap between the layers. There may be an interveninglayer, for instance a light transmissive binder/bonding layer, which isbonded to the wavelength selective filter layer and thephotoluminescence layer. Such an arrangement is still encompassed withinthe meaning of “deposing directly” and “in direct contact with” for thepurposes of this specification. By way of illustration only, the variouslayers in the figures are shown separated when they are not in directcontact with each other, that is where they are fabricated separatelyand then stacked together. When depositing the photoluminescencewavelength converting layer using screen printing, the lighttransmissive binder material can comprise for example a lighttransmissive UV-curable acrylic adhesive such as UVA4103 clear base fromSTAR Technology of Waterloo, Ind. USA. An advantage of depositing thephotoluminescence layer directly onto the filter layer is that this canincrease light emission from the backlight by eliminating an airinterface between the photoluminescence layer and filter layer. Such anair interface could otherwise lead to a greater probability of internalreflection occurring at the interface between the photoluminescencelayer and filter layer. The photoluminescence layer can be of athickness in a range 50 μm to 100 μm, more typically 40 μm to 80 μm.

In any embodiment, the photoluminescence layer can further incorporateparticles of a light scattering (diffusive) material, preferably zincoxide (ZnO). The light diffusive material can comprise silicon dioxide(SiO₂), titanium dioxide (TiO₂), magnesium oxide (MgO), barium sulfate(BaSO₄), aluminum oxide (Al₂O₃) or combinations thereof. Inclusion of alight scattering material can increase uniformity of light emission fromthe photoluminescence layer and can eliminate the need for a separatelight diffusive layer as are commonly used in color LCDs. Additionally,incorporating particles of a light scattering material with the mixtureof green- and red-emitting phosphor can result in an increase in lightgeneration by the photoluminescence layer and a substantial, up to 40%,reduction in the quantity of phosphor materials required to generate agiven color of light. Given the relatively high cost of phosphormaterials, inclusion of an inexpensive light scattering material canresult in a significant reduction in manufacturing cost for largerdisplays such a tablet computers, laptops, TVs and monitors. Furtherdetails of an exemplary approach to implement scattering particles aredescribed in U.S. Pat. No. 8,610,340 issued Dec. 17, 2013, which ishereby incorporated by reference in its entirety. The size of the lightscattering particles can be selected to scatter excitation lightrelatively more than light generated by the phosphor. In someembodiments, the light scattering material particles have an averagediameter (D50) of 200 nm of less, typically 100 nm to 150 nm.

Wavelength Converter—Wavelength Selective Filter Layer

Referring to FIG. 7A there is shown a schematic representation of awavelength selective filter layer (filter layer) 758. FIG. 7B is aschematic representation of the transmission and reflectioncharacteristics of a wavelength selective filter layer. As illustratedin the figures, the filter layer is substantially transmissive to bluelight 750 and substantially reflective to green light 762 and to redlight 764.

The filter layer 758 can comprise a thin-film dichroic filter(interference filter). Typically the filter layer comprises a film ofthickness 80 μm to 150 μm.

As indicated in FIG. 6 , the wavelength converter 654 is disposedbetween the light diffusive layer 652 and BEF 656 with the filter layer658 facing the light diffusive layer 652: that is the wavelengthconverter 654 is located in the optical path between the excitationsources 644 and LCD Display panel 102 (FIG. 1 ) with the filter layer658 facing the excitation sources 644. With such a configuration, thefilter layer 658 allows the free passage of blue excitation light 650 tothe photoluminescence layer 660 where a proportion of it will beconverted, through a process of photoluminescence, to greenphotoluminescence light 662 and red photoluminescence light 664. Due tothe isotropic nature of photoluminescence light generation, green light662 and red light 664 generated by the green- and red-emitting phosphorswill be emitted in all directions including directions towards theexcitation sources 644. However, due to the optical characteristics ofthe filter layer 658 such green and red light 662, 664 will be reflectedby the filter layer 658 back towards the Display panel (as shown in FIG.1 ).

Edge-Lit Backlight

While the backlight of the invention finds particular utility indirect-lit backlight arrangements, the backlight, in particularwavelength converter of the invention, also finds utility in edge-litbacklight arrangements.

FIG. 8 is a schematic cross-sectional representation of an edge-litcolor LCD 800 in accordance with an embodiment of the invention. FIG. 9is a schematic exploded cross-sectional representation of an edge-litbacklight 904. The backlight 904 comprises a light guide (waveguide) 966with one or more excitation sources 944 located along one or more edgesof the light guide 966. As indicated, the light guide 966 can be planar,though, in some embodiments, it can be tapered (wedge-shaped) forpromoting a more uniform emission of excitation light from a front lightemitting face 970 (upper face as shown in FIG. 9 that faces the DisplayPanel) of the light guide 966.

The excitation sources 944 are configured such that in operation, theygenerate blue excitation light 950 which is coupled into one or moreedges of the light guide 966 and then guided, by total internalreflection, throughout the volume of the light guide 966 and finallyemitted from the front light emitting face 970 of the light guide (upperface that faces the Display Panel). As shown in FIG. 9 , and to preventthe escape of light from the backlight 904, the rear face (lower face asshown) of the light guide 966 can comprise a light reflective layer(surface) 968 such as Vikuiti™ ESR (Enhanced Spectral Reflector) filmfrom 3M.

As with the direct-lit configuration of FIG. 6 , the wavelengthconverter 954 is disposed between the light diffusive layer 952 and BEF956 with the filter layer 958 facing the light diffusive layer 952: thatis the wavelength converter 954 is located in the optical path betweenthe excitation sources 944 and an LCD Display panel (as shown in FIG. 8) with the filter layer 958 facing the excitation sources 944. With sucha configuration, the filter layer 958 allows the free passage of blueexcitation light 950 to the photoluminescence layer 960 where aproportion of it will be converted, through a process ofphotoluminescence, to green photoluminescence light 962 and redphotoluminescence light 964. Due to the isotropic nature ofphotoluminescence light generation, green light 962 and red light 964generated by the green- and red-emitting phosphors will be emitted inall directions including directions towards the light guide 966.However, due to the optical characteristics of the filter layer 958 suchgreen and red light will be reflected by the filter layer 958 backtowards the Display panel (as shown in FIG. 8 ).

FIG. 10 is a schematic cross-sectional representation of a furtheredge-lit color LCD 1000 in accordance with an embodiment of theinvention. FIG. 11 is a schematic exploded cross-sectionalrepresentation of an edge-lit backlight 1104. As with the backlight ofFIG. 9 , the backlight 1104 comprises a light guide (waveguide) 1166with one or more excitation sources 1144 located along one or more edgesof the light guide 1166. As indicated, the light guide 1166 can beplanar, though, in some embodiments, it can be tapered (wedge-shaped)for promoting a more uniform emission of light from a front lightemitting face 1170 of the light guide (upper face as shown in FIG. 11that faces the Display Panel).

As shown in FIG. 11 , and to prevent the escape of light from thebacklight 1104, the rear face (lower face as shown) of the light guide1166 can comprise a light reflective layer (surface) 1168 such asVikuiti™ ESR (Enhanced Spectral Reflector) film from 3M.

In this embodiment, and as indicated in FIG. 11 , the wavelengthconverter 1154 is disposed on an edge face 1172 of the light guide 1166with the photoluminescence layer 1160 facing the edge face 1172 of thelight guide 1166: that is the wavelength converter 1154 is located inthe optical path between the excitation sources 1144 and light guide1166 (and thereby the LCD Display panel shown in FIG. 10 ) with thefilter layer 1158 facing the excitation sources 1144, and thephotoluminescence layer 1160 facing the edge face 1172 of the lightguide 1166. With such a configuration, the filter layer 1158 allows thefree passage of blue excitation light 1150 to the photoluminescencelayer 1160 where a proportion of it will be converted, through a processof photoluminescence, to green photoluminescence light and redphotoluminescence light. Due to the isotropic nature ofphotoluminescence light generation, green light and red light generatedby the green- and red-emitting phosphors will be emitted in alldirections including directions towards the excitation sources 1144.However, due to the optical characteristics of the filter layer 1158,such green and red light will be reflected by the filter layer 1158 backtowards the edge face 1172 of the light guide 1166. It will beappreciated that in this embodiment, white light 1140 is coupled intothe light guide 1166.

FIGS. 12 to 19 are schematic cross-sectional representations wavelengthconverters 1254 to 1954 respectively, for use in the color LCDs of FIGS.1, 8 and 10 .

As shown in FIG. 12 , the wavelength converter 1254 can further comprisea light transmissive protective layer 1274 on the photoluminescencelayer 1260. The light transmissive protective layer 1274 can comprise acoating of a light transmissive material, such as silicone, that isdeposited directly onto the photoluminescence layer 1260. Such aprotective layer 1274 can be of a thickness of about 10 μm and can bedeposited by screen printing or slot-die coating. In other arrangements,the protective layer 1274 can comprise a light transmissive film ofthickness 20 μm to 50 μm, such as PET (Polyethylene Terephthalate),which is laminated (bonded) to the photoluminescence layer. Such a lighttransmissive protective layer 1274 can provide protection to thephotoluminescence layer 1274 which can be particularly beneficial whenthe red photoluminescence material comprises a manganese-activatedfluoride phosphor such as KSF.

As shown in FIG. 13 , the wavelength converter 1354 can further comprisea light diffusive (Diffuser) layer 1376 on the photoluminescence layer1360. The light diffusive layer 1376 can comprise a coating of a lighttransmissive material, such as silicone loaded with light scattering(diffusive) particles, which is deposited directly onto thephotoluminescence layer 1360. The light diffusive layer 1360 canincrease the color uniformity and intensity uniformity of the whitelight generated by the backlight. Further, it has been found that theinclusion of a diffusing layer can reduce the phosphor amount requiredsignificantly (up to 60%) without affecting the quantum efficiency. Sucha significant reduction in phosphor usage provides a substantial costsaving; especially for large format displays since the phosphor layer isprovided over the entire display area.

As shown in FIG. 14 , the wavelength converter 1454 comprises aphotoluminescence layer 1460 and a filter layer 1458. In this embodimentthe photoluminescence layer 1460 comprises a multi-layered structurecomprising a first layer 1460G containing the green photoluminescencematerial (green photoluminescence layer) and a layer 1460R containingthe red photoluminescence material (red photoluminescence layer). Asindicated, the red photoluminescence layer 1460R is directly in contactwith and bonded to the filter layer 1458, that is the layer of the redphotoluminescence material is in closer proximity to the wavelengthselective filter layer than the layer of the green photoluminescencematerial. It will be understood that “closer proximity” is used tospecify that the layer of the red photoluminescence material is proximal(i.e. a proximal layer) to the wavelength selective filter layer, whilethe layer of the green photoluminescence material is distal (i.e. adistal layer) to the wavelength selective filter layer. Such anarrangement may be particularly beneficial when the redphotoluminescence material comprises a manganese-activated fluoridephosphor (such as KSF) whose absorption efficiency is significantlylower than that of a green photoluminescence material. In this way, theprovision of the red photoluminescence material, such as KSF, in arespective layer proximal (adjacent) to the wavelength selective filterlayer can, compared with a single-layered photoluminescence structure,improve luminous efficacy of the backlight and reduce the quantity ofred photoluminescence material required to achieve a comparable redemission characteristic. The wavelength converter 1454 can be fabricatedby directly depositing (fabricating), by for example screen printing,the red photoluminescence layer 1460R onto the wavelength selectivefilter layer 1458. Then, the green photoluminescence layer 1460G can bedirectly deposited (fabricated), by for example screen printing, on thered photoluminescence layer 1460R.

As shown in FIG. 15 , the wavelength converter 1554 is the same as thatshown in FIG. 14 , except the green photoluminescence layer 1560G andthe red photoluminescence layer 1560R are transposed such that the greenphotoluminescence layer 1560G is directly in contact with and bonded tothe filter layer 1558.

As shown in FIG. 16 , the wavelength converter 1654 is the same as thatshown in FIG. 14 , except that the multi-layered photoluminescence layer1660 is fabricated separately and then bonded to the wavelengthselective filter layer 1658. The photoluminescence layer 1660 can befabricated by fabricating a first layer, by for example extrusion,containing one of the red or green photoluminescence materials and theremaining layer deposited directly (fabricated) on the firstphotoluminescence layer by depositing by screen printing. Thisembodiment thus differs from FIG. 14 in that the photoluminescence layer1660 is bonded to the filter layer 1658 with an intervening lighttransmissive bonding layer 1678 therebetween. It will be understood thatthe since there is no air gap between the photoluminescence layer 1660and the filter layer 1658, the photoluminescence layer 1660 is disclosedas being directly deposited on or directly bonded to the filter layer1658, despite the presence of the light transmissive bonding layer 1678.This is because it retains the technical effect of the invention despitethe presence of one or more intervening layers.

As shown in FIG. 17 , the wavelength converter 1754 is the same as thatshown in FIG. 16 , except that there is an additional intervening lighttransmissive bonding layer 1680 located between the greenphotoluminescence layer 1760G and the red photoluminescence layer 1760R.In this embodiment, the green and red photoluminescence layers1760G/1760R are manufactured separately (for example by an extrusionprocess) before being bonded to one another by means of lighttransmissive bonding layer 1680. The resulting photoluminescence layer1760 is bonded to the filter layer 1758 to fabricate the wavelengthconverter 1754. In this embodiment, the red photoluminescence layer1760R is bonded to the filter layer 1758, despite the presence of thelight transmissive bonding layer 1778.

As shown in FIG. 18 , the wavelength converter 1854 is the same as thatshown in FIG. 14 , except a light transmissive protective layer 1874 isdeposited on top of the green photoluminescence layer 1860G on the sidefacing away from the filter layer 1858. Such a light transmissiveprotective layer 1874 can be of a thickness of about 10 μm and can bedeposited by screen printing or slot-die coating. In other arrangements,the light transmissive protective layer 1874 can comprise a lighttransmissive film of thickness 20 μm to 50 μm, such as PET (PolyethyleneTerephthalate), which is laminated to the photoluminescence layer 1860.

As shown in FIG. 19 , the wavelength converter 1954 is the same as thatshown in FIG. 14 , except a light diffusive layer 1976 is deposited ontop of the green photoluminescence layer 1960G on the side facing awayfrom the filter layer 1958. The light diffusive layer 1976 ensuresuniform illumination of the wavelength converter 1954 with blueexcitation light (not shown).

As used in this document, both in the description and in the claims, andas customarily used in the art, the words “substantially,”“approximately,” and similar terms of approximation are used to accountfor manufacturing tolerances, manufacturing variations, manufacturingimprecisions, and measurement inaccuracy and imprecision that areinescapable parts of fabricating and operating any physical object.

It will be appreciated that the present invention is not restricted tothe specific embodiments described, and that variations can be made thatare within the scope of the invention.

REFERENCE NUMERALS

-   00 Color LCD-   02 LC Display Panel-   04 Backlight-   06 Front plate-   08 Back plate-   10 Liquid Crystal (LC)-   12 Glass plate-   14 Viewing face-   16 First polarizing filter layer-   18 Anti-reflective layer-   20 Color filter plate-   22 Light transmissive common electrode plane-   24 Red sub-pixel filter element-   26 Green sub-pixel filter element-   28 Blue sub-pixel filter element-   30 Unit pixel-   32 Opaque divider/black matrix-   34 Glass plate-   36 TFT-   38 Second polarizing filter layer-   40 white Light-   42 Backlight light emitting face-   44 Excitation source-   46 Enclosure Floor-   48 Enclosure-   50 Blue excitation light-   52 Light diffusive layer-   54 Wavelength converter-   56 Brightness Enhancement Film (BEF)-   58 Wavelength selective filter layer-   60 Photoluminescence wavelength converting layer (photoluminescence    layer)-   62 Green photoluminescence light-   64 Red photoluminescence light-   66 Light guide-   68 Light reflective layer-   70 Light emitting face of light guide-   72 Edge face of light guide-   74 Light transmissive protective layer-   76 Light diffusive layer-   78 Light transmissive bonding layer-   80 Light transmissive bonding layer

What is claimed is:
 1. A display backlight, comprising: an excitationsource for generating blue light; and a wavelength converter being aunitary construction comprising a combination of a wavelength selectivefilter layer bonded to a photoluminescence layer, wherein thephotoluminescence layer comprises a green photoluminescence material anda red photoluminescence material; and wherein the wavelength selectivefilter layer is transmissive to blue light and is reflective to greenand red light.